Various measures for reduction of emissions of CO2 are being taken in order to deal with atmospheric pollution and global warming. In particular, in the automobile industry, the reduction of emissions of CO2 is highly expected in association with the spread of electric vehicles and hybrid electric vehicles. Thus, development of high-performance secondary batteries serving as driving power sources of motors for use in such vehicles, is actively being carried out. Since a higher capacity and cycle property are particularly required for the secondary batteries for driving motors, lithium ion secondary batteries having high theoretical energy are gaining increasing attention among other types of secondary batteries.
The lithium ion secondary batteries are required to store a large amount of electricity in positive electrodes and negative electrodes by unit mass, in order to increase energy density of the lithium ion secondary batteries. The lithium ion secondary batteries highly depend on active materials used in the respective electrodes to fulfill such a requirement.
As one of measures to improve performance of lithium ion secondary batteries, there is known a negative electrode active material capable of ensuring a high capacity and releasing stress caused by expansion and contraction, and a battery using such a negative electrode active material, as proposed in Patent Literature 1. The negative electrode active material contains Si as a first element, Ge as a second element, and at least one of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and Nb as a third element. Patent Literature 1 discloses that the content of Ge is in the range from 5 to 12 atom %, and the content of the third element is in the range from 0.5 to 10 atom %, with respect to the content of Si.